The long-term goal of this research is to understand the mechanisms by which Kit ligand (Kitl) promotes the development of diverse cell types in mammnals. Kitl is the ligand for the Kit receptor tyrosine kinase and is a member of the short chain group of helical cytokines. In humans, Kit mutations cause gastrointestinal tumors, hematopoietic proliferative disorders and piebald trait. In mice, Kitl is encoded by the Steel (Sl) locus and studies of Kitl mutants have established that Kitl is essential to the embryonic and postnatal development of hematopoietic cells, germ cells and melanocytes. We propose to use a series of Kitl(sl) mutations, including eight newly characterized mutations, to genetically dissect Kitl function. Our previous studies have characterized the effects of these mutations on mouse survival, hematopoietic cells and germ cells and have identified mutations that exert strong and weak effects on the in vivo phenotype. We will use molecular and genetic approaches to determine how these mutations affect Kitl function and to gain a better understanding of the Kitl/Kit signaling pathway. In Specific Aim 1, we will use an in vitro system to examine the mechanism of altered function of these Kitl(sl) mutants. We will determine the intracellular processing and cell surface expression of Kitl(sl) mutants and determine if Kitl(sl) mutants are deficient for binding to Kit, for activation of Kit and for in vitro bioactivity: In Specific Aim 2, we will use comparative molecular modeling to predict the effects of Kitl mutations on Kitl structure. In Specific Aim 3, we will use selected Kitl(sl) mutants to dissect further the role of Kitl in three phases of germ cell development in embryos: during emergence of primordial germ cells (PGCs) from the epiblast; during migration of actively dividing PGCs of mid-gestation; and during the post-migratory phase of late gestation. In Specific Aim 4, we will genetically characterize a novel modifier of the Kitl about pigmentation phenotype in mice. By generating a new congenic strain, we will test whether this pigmentation modifier affects other aspects of the Kitl(sl) mutant phenotype. Ultimately, the gene responsible for this modifier will be identified and will provide new information about the Kitl signaling pathway. Together, these studies will provide new insights into Kitl function and may lead to novel strategies for the development of more effective cytokines.